Photolithography involves transferring patterns of small features by passing light through a mask onto a light sensitive surface. When photolithography is used to create nested lines and spaces that are close together and repeating, the passage of light through the photomask can create diffraction effects that distort the pattern transferred. Diffraction is the bending of light as it passes through a slit or by an obstruction that is physically the approximate size of, or smaller than, the light's wavelength. The diffraction effects that occur as a result of light passing through a photomask are dependent on the spatial frequency of the photomask's features.
Spatial frequency is the number of times a mask feature is repeated in a given space. Spatial frequency is the inverse of pitch. As shown in FIG. 1, a photomask with a square grid has a spatial frequency that differs depending on the axis used for measurement. When the spatial frequency of the square grid is measured parallel to the sides of the square, the spatial frequency is equal to 1/p. The variable p is the length of one side of a square, which is also the pitch in this case. However, when the spatial frequency is measured along the diagonal of the square grid, the spatial frequency is equal to 1/(1.414*p). The pitch is equal to 1.414*p, or the length of a diagonal of one of the square features in the grid. Thus, the pitch and the spatial frequency of a photomask with a square grid are different when measured along the diagonal, causing anisotropic optical diffraction behavior.
For these reasons, it is difficult to lessen the diffraction effects of a photomask with a square grid because there is more than one spatial frequency to consider. The present invention provides an assist pattern to mitigate the anisotropic optical diffraction behavior. The assist pattern includes additional holes in the photomask which are angularly offset from corresponding holes of a base pattern.